1. Field of the Technology
The present application relates generally to data communications for mobile communication devices. More particularly, the application relates to unacknowledged Network Layer Service Access Point Identifier (NSAPI) recovery for Sub-Network Dependent Convergence Protocol (SNDCP) communication in General Packet Radio Service (GPRS) networks.
2. Description of the Related Art
A mobile station (MS) may communicate in standardized networks such as Global System for Mobile (GSM) or General Packet Radio Service (GPRS) networks. In order to communicate data and voice, a MS may need to operate in accordance with both GSM and GPRS technologies. A Class B MS is a dual mode MS that can communicate in either GSM mode for voice or in GPRS mode for data—but not in both modes simultaneously. In GPRS mode, a Class B MS depends on protocols layered as a stack; network infrastructure nodes have corresponding stacks to thereby enable communications between the MS and various nodes and networks. 3rd Generation Partnership Project (3GPP) standard document 24.007 (European Telecommunications Standards Institute (ETSI) Technical Specification (TS) 124 007 V4.1.0 (2001-12)) section 5.2 describes the GPRS protocol stack architecture diagram. GSM and GPRS share certain lower protocol layers, such as the GSM radio frequency (RF) layer.
GSM and GPRS techniques are known to those skilled in the art, as well as are some of their deficiencies. One such deficiency is related to the resetting of one of the protocol stack layers, namely, the Logical Link Control (LLC) layer. GSM standard document 04.464 (ETSI TS 101 351 V8.3.0 (2000-03)) section 8.5.3.1 details the negotiation of parameter Reset in the LLC. There, it explicitly specifies that the LLC shall discard requests pending from layer-3 to logical link entities (LLEs) with no further action.
Unfortunately, important user information may be lost as the LLC discards the pending requests. This happens particularly when using unacknowledged Network Layer Service Access Point Identifier (NSAPI) communication over Sub-network Dependent Convergence Protocol (SNDCP) communications. 3GPP standard document 04.65 (ETSI TS 101 297 V8.2.0 (2001-09)) section 5.1.2.1 describes what the GPRS SNDCP layer must do when it receives a logical link reset indication (LL.RESET.indication). Note that the standard is very explicit about what the SNDCP should do upon receiving the LL reset indication if the NSAPI is using acknowledged LLC operation.
The fact that a Class B MS can operate only in one mode at a given time (either GSM or GPRS mode) makes a Class B MS particularly vulnerable to this LLC reset deficiency. In particular, an LLC reset occurs when changing from the GPRS mode to the GSM mode. The process of changing modes from data to voice and back can occur on many occasions during the normal use of a MS (e.g. due to network coverage inadequacies), occasionally causing data to be lost within the MS stack over the time of operation of the MS. The GPRS and GSM standards remain silent on how to recover from adverse effects of such mode changes, resulting in some implementations of the standard exhibiting data loss at a MS. As the ability to communicate data decreases, so does the utility of the MS. The time of operation of the MS can be drastically curtailed by improperly losing data within the protocol layers of the MS stack.
Accordingly, there remains a need for a system and method of unacknowledged NSAPI recovery in SNDCP communication that overcomes the limitations present in the current Class B MSs which cooperate with existing standards. There remains a further need for a system and method of unacknowledged NSAPI recovery in SNDCP communication that systematically recovers from transitions between GPRS and GSM modes in a Class B MS.